UX: the ability to hide the admin header (#30175)

Some pages like new/edit item should not display admin header. New attribute called `@shouldDisplay` was added.

As a proof of concept, the flags page was updated.

spec/system/admin_flags_spec.rb

@@ -27,7 +27,7 @@ describe "Admin Flags Page", type: :system do
     )
 
     admin_flags_page.visit
-    expect(page).to have_css(".admin-page-header")
+    expect(admin_header).to be_visible
 
     admin_flags_page.toggle("spam")
     topic_page.visit_topic(post.topic).open_flag_topic_modal
@@ -81,7 +81,9 @@ describe "Admin Flags Page", type: :system do
     expect(admin_flags_page).to have_add_flag_button_enabled
 
     admin_flags_page.click_add_flag
-    expect(page).not_to have_css(".admin-page-header")
+
+    expect(admin_header).to be_hidden
+
     admin_flag_form_page
       .fill_in_name("Vulgar")
       .fill_in_description("New flag description")
@@ -113,7 +115,7 @@ describe "Admin Flags Page", type: :system do
 
     # update
     admin_flags_page.visit.click_edit_flag("custom_vulgar")
-    expect(page).not_to have_css(".admin-page-header")
+    expect(admin_header).to be_hidden
     admin_flag_form_page.fill_in_name("Tasteless").click_save
 
     expect(admin_flags_page).to have_flags(

spec/system/admin_user_fields_spec.rb

@@ -6,9 +6,11 @@ describe "Admin User Fields", type: :system do
   before { sign_in(current_user) }
 
   let(:user_fields_page) { PageObjects::Pages::AdminUserFields.new }
+  let(:admin_header) { PageObjects::Components::AdminHeader.new }
 
   it "correctly saves user fields" do
     user_fields_page.visit
+    expect(admin_header).to be_visible
     user_fields_page.add_field(name: "Occupation", description: "What you do for work")
 
     expect(user_fields_page).to have_user_field("Occupation")
@@ -30,6 +32,8 @@ describe "Admin User Fields", type: :system do
     user_fields_page.visit
     user_fields_page.click_add_field
 
+    expect(admin_header).to be_hidden
+
     form = page.find(".user-field")
     editable_label = I18n.t("admin_js.admin.user_fields.editable.title")
 
@@ -69,6 +73,8 @@ describe "Admin User Fields", type: :system do
 
       form.find(".user-field-name").fill_in(with: "Favourite Transformer")
 
+      expect(admin_header).to be_hidden
+
       form.find(".btn-primary").click
 
       expect(page).to have_no_text(I18n.t("admin_js.admin.user_fields.requirement.confirmation"))

spec/system/page_objects/components/admin-header.rb

@@ -6,6 +6,14 @@ module PageObjects
       def has_tabs?(names)
         expect(page.all(".admin-nav-submenu__tabs a").map(&:text)).to eq(names)
       end
+
+      def visible?
+        has_css?(".admin-page-header")
+      end
+
+      def hidden?
+        has_no_css?(".admin-page-header")
+      end
     end
   end
 end